1. Field of the Invention
The present invention generally relates to apparatus for recording information in a disk-shaped record medium in which information can be recorded and from which the recorded information can be read-out and for reproducing the same therefrom and, more particularly, to a disk recording and reproducing apparatus using two laser beams.
2. Description of the Prior Art
As an information record medium, a so-called write once optical disk is proposed. According to this write once optical disk which is already available as a product information can be recorded thereon and reproduced therefrom by using an optical or laser beam. Of such write once optical disks, a magneto-optical disk is known, in which information can be repeatedly written. Such magneto-optical disk is comprised of a substrate, a vertical magnetization layer which forms a recording layer on the surface of the substrate and a protecting layer protecting the vertical magnetization layer. This magneto-optical disk is generally shaped as a disk. Information is written in the vertical magnetization layer on a recording track forming portion of the disk to record the information and the information written in the vertical magnetization layer on the recording track forming portion is read-out to reproduce the information.
When information is written in the magneto-optical disk, the magneto-optical disk is rotated about its central portion at a predetermined revolution speed. Under the condition that a predetermined external magnetic field acts on the vertical magnetization layer on the recording track forming portion, a laser beam is introduced onto the vertical magnetization layer, and the vertical magnetization layer on the recording track forming portion at its portion illuminated with the laser light beam is magnetized in the direction corresponding to the direction of the external magnetic field in accordance with the resultant rise of temperature. Then, if the magnetic field modulation recording system is employed in which the external magnetic field is changed in response to recording information under the condition that an intensity of laser beam is substantially constant or the optical modulation recording system is employed in which the laser light beam is changed in intensity in response to a recording information under the condition that the external magnetic field is substantially constant, magnetization direction inverting regions of a predetermined pattern are formed on the vertical magnetization layer on the recording track forming portion, the information being thereby recorded on the magneto-optical disk.
When the information recorded on the magneto-optical disk is read-out, a laser light beam of intensity less than that of the recording laser light beam is introduced onto the vertical magnetization layer on the recording track forming portion and a reflected laser light beam from the vertical magnetization layer on the recording track forming portion is detected by a photodetecting section so that the photo-detecting section derives a read-out signal corresponding to the information written in the magneto-optical disk. When the information is read-out as described above, the reflected laser light beam from the vertical magnetization layer on the recording track forming portion produces a rotation of a polarizing plane in response to the magnetization direction inverting pattern in the vertical magnetization layer corresponding to the written information due to the Kerr effect. Such rotation of polarizing plane is detected by detecting and comparing respective orthogonal polarized components of, for example, the reflected laser light beam, and a read-out signal is formed on the basis of a compared output which is changed in response to the rotation of the polarizing plane.
In the disk recording and reproducing apparatus for recording an information on the magneto-optical disk or reproducing the information from the magneto-optical disk, when the magneto-optical disk is rotated at the predetermined rotational speed and the laser light beam becomes incident on the vertical magnetization layer of the record track forming portion under the condition that the predetermined external magnetic field is applied to the vertical magnetization layer on the recording track forming portion thereby to record a new information on the magneto-optical disk, it is desirable to monitor the information being recorded in the magneto-optical disk in order to confirm the recorded condition of the information. Further, when the information already recorded on the magneto-optical disk is partially varied, corrected or new information is additionally recorded on the magneto-optical disk, a so-called search playback is required to read-out the information recorded on the magneto-optical disk at its recording position preceding the recorded position on the recording track in order to detect the position at which the additional recording or the like is carried out.
To this end, the following disk recording and reproducing apparatus is proposed.
According to this conventional disk recording and reproducing apparatus, when new information is recorded on the magneto-optical disk or the information already recorded on the magneto-optical disk is partially varied, corrected or when new information is additionally recorded on the magneto-optical disk, two laser light beams of an information writing laser light beam and an information reading laser light beam are respectively emitted from two laser light emitting elements and introduced into the magneto-optical disk independently. When new information is recorded on the magneto-optical disk, the information written in the vertical magnetization layer on the recording track forming portion of the magneto-optical disk on the basis of the information writing laser light beam is read-out from the recording track forming portion of the magneto-optical disk by the information reading laser light beam thereby to obtain a reproduced information signal, and the information recorded on the magneto-optical disk by the information writing laser light beam is monitored on the basis of the above-described reproduced information signal. Further, when the information already recorded on the magneto-optical disk is partially varied, corrected or an information is additionally recorded on the magneto-optical disk, the so-called search playback is performed on the information recorded on the vertical magnetization layer on the recording track forming portion of the magneto-optical disk by the information reading laser light beam thereby to obtain a reproduced information signal. Then, a position at which the recorded information should be partially changed, corrected or the new information should be additionally recorded by the use of the information writing laser beam is detected on the basis of such reproduced information signal.
However, the disk recording and reproducing apparatus in which the two laser light beams of the information writing laser light beam and the information reading laser light beam are respectively generated from the two laser emitting elements and are independently introduced onto the magneto-optical disk unavoidably encounters the following problems. That is two optical systems including two laser emitting elements and control sections for the respective optical systems are needed in order to generate and control the two laser light beams of the information writing laser light beam and the information reading laser light beam, which necessitates a complicated arrangement of optical system and also a complicated arrangement and operation of the control system. Furthermore, there is the substantial disadvantage that a focusing control and a tracking control are not carried out in a stable manner.
In order to remove the aforementioned shortcomings, the assignee of the present application has proposed the following improved disk recording and reproducing apparatus (see Japanese Patent Application No. 63-327357).
According to this previously-proposed disk recording and reproducing apparatus, information is recorded by writing an information in recording tracks of a rotating magneto-optical disk and an information is reproduced by reading-out an information written in the recording tracks of the rotating magneto-optical disk. One light beam from a light beam generating source is split to provide at least two light beams and two light beams are respectively introduced into the rotating magneto-optical disk adjacent two recording tracks, whereby information is recorded in the recording track on which one of the two light beams introduced into the magneto-optical disk becomes incident, while recorded information is reproduced on the basis of a detected output which results from detecting the change in which the other light beam of the two light beams incident on the magneto-optical disk is affected by the recording track. Therefore, the new information recorded in the magneto-optical disk can be monitored and the search playback for partly changing or correcting the information recorded on the magneto-optical disk or for performing additional recording can be performed with ease by an inexpensive optical system of a relatively simple arrangement which utilizes a single light beam generating source and a control system therefor.
As described above, in the disk recording and reproducing apparatus in which one of the two light beams, provided by splitting one light beam from the laser beam generating source, is related to the recording of information on the magneto-optical disk and the other light beam is related to the reproduction of information from the magneto-optical disk, a special problem will not occur if the information is recorded on and reproduced from the magneto-optical disk under the condition that the magneto-optical disk is rotated at a constant rotational speed regardless of the position at which the information recording light beam becomes incident on the radial direction of the magneto-optical disk. However, if the information is recorded on and reproduced from the magneto-optical disk under the condition that the magneto-optical disk is rotated at a rotational speed this is changing in response to the incident position of the information recording light beam on the radial direction of the magneto-optical disk so as to provide a constant relative moving speed between the recording track and the information recording light beam incident on the recording track, that is, so as to cause the information recording light beam to be introduced onto the recording track at a constant linear velocity, the information is read-out from the recording track of the magneto-optical disk by an information reproducing light beam at a linear velocity different from a linear velocity of the information recording light beam used to record the information on the recording track upon recording, that is, at a linear velocity of the information reproducing light beam relative to the recording track. There is then the substantial risk that the information cannot be properly reproduced from the magneto-optical disk on the basis of the detected output which results from detecting the information reproducing light beam.